Self-temperature regulating heater cables formed with positive temperature coefficient (PTC) of resistance characteristics are known in the art. Typically, such cable is made with a PTC composition formed of a polymeric matrix through which a carbon black is distributed. A PTC composition is usually formed by commencing with a mixture of a desired carbon black with a copolymer capable of blending with carbon black powder. A polyethylene polymer is then added and mixed with the blended carbon black and a filler material such as a flame retardant. The final blend is then formed into pellets for subsequent use in an extruder to extrude a layer of the PTC material around one or several conductors.
The specific composition of the PTC material is often selected to meet particular criteria for the PTC material or to enhance the efficiency of its manufacture. One area of interest has addressed the separate heating or annealing step that is often required to achieve a desired lower resistance in the PTC material. After annealing, the PTC material is subjected to a cross-linking step by way of an irradiation treatment to stabilize the composition.
For example, in U.S. Pat. No. 4,277,673 to Kelly, PTC compositions are described with which annealing times are reduced by selecting a high resistivity carbon black. The annealing time is stated to be reduced from 64 hours, when a conductive carbon black such as Cabot Corporation's Vulcan XC72 is used with a polyethylene such as DFD 6005, down to five hours when a Cities Service Co.'s (Columbia Chemical) Raven 1255 carbon black is used. There are many different carbon blacks available from commercial sources such as The Cabot Corporation under its trademarks Black Pearl, Vulcan, Monarch, Regal and Elftex or The Columbian Chemical Corporation under its trademark Raven, and from many other companies. Performance data on these carbon blacks are published with various characteristics.
Further reductions in anneal times have been achieved as described in U.S. Pat. No. 4,668,857 to Smuckler. This patent describes a conductive polymeric carbon black composition using a polymer with a melt flow index of at least 1.0 in order to achieve desired conductivities with either no annealing or with as short an annealing time as from one to five minutes.
U.S. Pat. No. 4,818,439 to Blackledge et al. proposes a carbon black loaded polymer material with which annealing is obtained in the short travel time from the extrusion head to a quenching water trough. An anneal time as short as about 42 milliseconds is described as achieving a desired conductivity when the polymeric matrix of the PTC composition is an olefine polymer having a low average molecular weight and a high proportion of molecules having a number average molecular weight below 23,000, generally requiring blends of polymeric materials. In this patent a control PTC composition, using DFD 6005 polyethylene made by Union Carbide Corporation as the polymeric matrix, is described as requiring an unacceptably long anneal time to develop the required PTC properties, e.g., from one to three minutes of annealing.